Due to the rise in environmental consciousness, a particulate filter is utilized to remove particulate matters from exhaust gas of a diesel engine. Since the efficiency of the particulate filter decreases gradually after a period of use, a regeneration process for the particulate filter is required when the amount of the particulate matters accumulated in the particulate filter exceeds a certain threshold. Ideally, the particulate matters in the articulate filter are supposed to be completely combusted in the regeneration process without leaving residue in the particulate filter. However, the regeneration process may be adversely affected by various factors, e.g., lack/excess of fuel introduced into the particulate filter, inappropriate combustion temperature, etc., thereby leading to incomplete combustion of the particulate matters. Furthermore, residual ash resulting from combustion of the particulate matters would accumulate in the particulate filter after the regeneration process and the presence of residual ash adversely affects efficiency of the particulate filter.